After you eat a meal because you don't want the food to sit in between your teeth. Food that sits in between your teeth causes cavities and bad breath overtime.
For me, it's at night during my night time routine. It gives me something to accomplish to end the day on a great note.
However, I think after supper would be the best for my teeth.
I usually floss in the evening, and if food gets thoroughly stuck, then directly after a meal. The reason is that while sleeping, the mouth usually dries, leading to less saliva for bacterial protection. Especially if you have starchy/sugary foods lodged in your teeth, these spots will lead to accelerated tooth decay.